Method of making contact wires



.Oct. 21, 1947. I R. A. EHRHARDT ET AL. 2,429,222

METHODS OF MAKING CONTACT WIRES Filed June 5, 1943 2, Sheets-Sheet l bF/af 2 FIG 3 FIG. 4

R ,4. EHRHARDT m R. a. HUMPHREY ATTORNEY Oct. 21, 1947. R. A. EHRHARDTETAL 2,429,222

METHODS OF MAKING CONTACT WIRES Filed June 5,. 1943 2 Sheets-Sheet 2 R,4. EHRHAPDT WVENT-ORS R a. HUMPHREV A TTORNEK Patented Clef. 21, 1947UNITED STATES METHOD OF MN G CONTACT WIRES Robert A. Ehrhardt,

New Providence, and Robert G. Humphrey, Madison, N. J assignors to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application June 5, 1948, Serial No. 489,744

Claims.

This invention relates to translating materials and devices andparticularly to methods of making them.

The objects are to reduce the cost of manufacturing contact wires fortranslators; to improve the electrical contact between the contact wireand its supporting member in the assembly; to facilitate the joinder ofthis wire and the supporting member by soldering or otherwise; tofacilitate the forming of the wire into the requisite shapes; and inother respects to improve these devices and the methods of making them.

The successful extension of radio signaling into the range of ultra-highfrequencies, corresponding in some cases to wavelengths of only a fewcentimeters, has depended in large measure upon the development of asuitable device for detecting, converting, translating, or otherwiseutilizing the signal waves at these extreme frequencies. For thispurpose the translating device most commonly used is one of thepoint-contact type in which the free end of a fine'contact wire of somematerial such as tungsten makes a point contact with the surface of arectifying element such as a silicon crystal. The other end of this wireis generally soldered 'to a metal support or holder forming a part ofthe translator assembly, and the minute character of the currentsflowing in the wire makes it imperative that the joint it forms with theholder shall be substantially perfect. This is true for two reasons:"any imperfection in the joint introduces a loss in the working current,which is serious in view of its small magnitude; and a faulty joint alsogives rise to noise currents, which are likewise objectionable. Sincethe fine tungsten wire cannot be soldered readily to the metallicholden'the present practice is to coat one end of the contact wire bydipping it in a hotbath of contact metal before performing the solderingoperation. This method is slowand expensive and, is very unsatisfactorywhere quantity production is required. Moreover, the

adherence between the tungsten wire and the coating is not sufiicient togive an adequate electrical contact.

In accordance with the present invention the objections above mentionedare overcome and an improved contact wire and joint are obtained by anew method of preparing, plating, and treating the wire before it issoldered or otherwise secured to the metal holder. vMore specifically,this new method comprises a series of steps which are so arrangedsequentially that the wire stock may be run continuously fromv thebeginning to the end of the process. Y The first step is to subject thetungsten wire to a cleaning operation, following which it is given astrike coating of nickel in a special bath. Following the strike coatingthe wire is given a heavier plating of nickel in an electroplating bathof suitable materials. Thereafter the wire proceeds through a finalelectrolytic bath in which it is given a coating of precious metal, suchas gold. The next step is to run the plated wire through aheatingchamber to cause a partial diffusion between the nickel and tungsten andbetween the nickel and gold. Thereafter the prepared wire is cleaned andis then ready for use in the rectifier assemblies.

In addition to the manufacturing advantage realized from this improvedprocess the contact wires prepared in this way also have distinctelectrical advantages. The layer of nickel adheres to the tungstensurface and forms a base for the outer layer of gold, which cannot bemade to adhere directly to the tungsten surface, and the heat treatmentthen forms an intimate union by alloying a part of the nickel with thetungsten and also by alloying a part of the nickel with the gold. Thus asubstantially perfect electrical joint is formed between the tungstenwire and the outer plating to which the soldered connection is made.Moreover, the outer layer of gold, which is highly resistant tooxidation, affords a superior surface to which a soldered joint havingthe requisite electrical and mechanical characteristics can be made.Another advantage of this method is that the strong adherence betweenthe coating and the tungsten, resulting from the diffusion of the metalsduring the heat treatment, makes it possible to bend and form the wireinto desired shapes without causing the coating to flake or peel. Theformation of a completely adherent coat'is essential not only at thesoldered to the holder but also in the intermediate section which mustbe formed and shaped to give the required resilience to the pointcontact, Any loosening of the coating on the tungsten sur face due tobending results in electrical disturbances in the working circuits.

These and other features of the invention will be discussed more fullyin the following detailed specification.

In the drawings accompanying the specificae tion:

Fig. 1 illustrates a length of tungsten wire (greatly enlarged) used asa point-contact element in a rectifier;

Fig. 2 shows a metallic cylinder of brass or other suitable metal whichserves as a holder for the contact wire; L

end of the wire where it is I Fig. 3 shows the contact wire fixed inposition in the metal holder;

Fig. 4 illustrates one of the rectifier assemblies; and

Fig. 5 shows the apparatus for performing the plating and heat-treatmentoperations on the contact wire.

A better understanding of the problem with which applicants invention isconcerned may be had by considering the small dimensions of some of thecomponent parts of these point-contact rectificrs and the minute currentwhich they conduct. The contact wire I, the free end of which makes aminute point contact with the surface of a silicon crystal '2 is about.203 inch in length and is as small as .005 inch in diameter. Thesilicon crystal 2 is about .OQO inch in diameter and has a thickness of.015 inch. The free end of the wire I is ground to a point in order toreduce the area of the rectifying engagement between the wire and thecrystal surface. In view of the minute size and delicate character ofthe point contact thus formed only currents of very small amplitude flowthrough the contact wire to the associated parts of the rectifierassembly.

The contact wire I is supported by a cylindrical holder 3, preferably ofbrass, to which it is attached by means of a soldered joint asillustrated in Fig. 3. As mentioned above, the difficulty in themanufacture of these devices is to obtain a joint between the wire I andthe holder 3 which is strong mechanically, free from noise sources, andoffers substantially no resistance to the flow of the minute currentsbetween the wire and the holder. Moreover, it is necessary to obtainsuch a joint without interfering with the ability of the wire to undergoflexing and bending in order that the resilient formation shown in Fig.3 may be attained. The process of preparing a contact wire which meetsall of these requirements will now be described in detail.

Referring to Fig. 5, the tunsten wire stock is taken from a supply reel4 and advanced in a continuous movement at a uniform rate of speed byany suitable driving means through a succession of electrolytic bathsand finally through a heat-treating apparatus from which it is deliveredto a storage reel 5. The linear speed of the moving wire l2-maybechosen-at any desired value, but it has been found that a speed of 2.5feet per minute gives good results. After the wire leaves-the supplyreel 4 it passes over a pulley 6 into a cleaning bath 1 around pulleys 8and thence out of the cleaning bath to the pulley 9. The bath lcomprises a twenty per cent solution of sodium hydroxide, and itspurpose is to give the wire a preliminary cleaning. To this end themoving wire is made the anode of theelectrolytic cell by connecting thepositive pole of generator it to the metal pulley 6 and by connectingthe negative pole of the generator to thesubmerged cathode H. Theproportions of the bath '1 relative to the speed of the moving wire maybe chosen to give the desiredtime interval of exposure. Applicants havefound that an interval of. about twenty=seconds at a current density of190 amperes per square foot is sufficient for the initial, cleaningstep. On emerging from the cleaning bath 1 the wire passes over thepulley 9 and. into a rinsing bath l3.

From the rinsing bath l3 the wire passes through the initial platingbath M. This bath comprises a special solution of nickel chloride andhydrochloric acidin the following proportions:

4 NiCl2.6I-I2O ounces per gallon 32 E01 fiuid ounces per gallon... 16

The moving wire is made the cathode by connecting the negative pole ofthe generator l5 to the metal pulley l6, and the positive pole of thegenerator is connected to the immersed anode ll. This bath is highlyacid and exerts a cleaning influence on the tunsten due to the copiousevolution of hydrogen. In this manner all traces of oxide or otherimpurities are removed from the surface of the wire, exposing the cleantungsten to the bath. The interval of immersion is relatively brief,preferably of the order of twentyfive seconds, but during this time thecleaning action takes place and a strike coating of nickel is depositedon the cleaned surface of the wire.

A current density of 720 amperes per square foot during the twenty-fivesecond exposure is found to give excellent results. The main purpose ofthis initial plating step is to lay a primary coat of nickel on'theclean tunsten surface as a base for the deposit of succeeding andheavier plates. After leaving the striking bath M the wire is againrinsed in a cleaning bath is.

From the cleaning bath Hi the wire proceeds into the nickel-plating bathL9 for the purpose of depositing a plate of nickel of substantialthickness. To this end the size of the bath I9 is chosen with referenceto the speed of the wire to give a timeinterval exposure of about sixtyseconds. One method of accomplishing this longer exposure is to loop themoving wire over a number of pulleys 2|], 2|, 22. The bath 19 mayconsist of any suitable and well-known nickel-plating solution such asthose including nickel chloride, nickel sulphate and looric acid. Thecurrent density in the bath i9 is maintained at '7 amperes per squarefoot by the generator 23, which has its negative pole connected to themetal pulley 124 in contact with the wire and its positive poleconnected to the submerged anode 25. From the plating bath H) the wireis guided into. another cleaning bath 26.

From the cleaning bath 26 the wire is ledinto a gold-plating bath 21 fordepositing a final plate of gold. It is desired to have the thickness ofthe gold plate considerably greater than that of the preceding nickelplate, and for this purpose the bath 2'! is designed to expose themoving wire to the solution for an interval of about two hundred andforty seconds. Any suitable and well-known gold plating solution may beemployed such as the following:

Ounces per gallon NaAuCNz l Na N M 1 Natal-IP04 1 Thecurrent densityofthe bath is maintained at l51amperes per square foot by means of thegenerator 2B, the negative pole ofwhich is connected to themoving wirethrough the medium of pulley-29 and thepositive pole of which isconnected-to the submergedanode- 30. Following the gold-plating bath thewire is cleaned in a bath 3i whereupon itis delivered to a temporarystorage receptacle 32. e

The finalheat treatmentstep ofthe process is performedby running theplated wire [2 through a chamber 33-containing purified hydrogensupplied from a tank 52. The chamber 33 is heated from a suitable sourceand is closely regulated tothe desired temperature, which may be about710C. The movement-of the wire through the hydrogen atmosphere ispreferably at a conas the driving member for siderably higher speed,about fourteen or fifteen feet per minute. This difference in speedbetween the movement of the wire through the successive baths andthrough the heat-treating chamber makes it necessary to introduce thestorage receptacle 32 for storing a quantity of the plated wire beforethe heat treatment is applied. If desirable a timing switch 41 may bearranged to start the motor after the plating mechanism has run longenough to accumulate a quantity of the plated wire. As soon as nearlyall the plated wire has been removed from receptacle 32 it engages theswivel contact 49 and closes a circuit for relay 50, which stops themotor 48 to allow a new supply of wire to accumulate.

After the treated wire passes from the chamber 33 it is subjected to thefriction of cloth pads 35 and 36 which are pressed together by a forceof approximately 16 pounds. As the wire leaves the pressure pads 35 and36 it is received and wound on the storage reel 5.

It will be understood that any suitable means may be employed for movingthe wire through the successive baths and through the heat-treatingchamber. A number of pulleys have been illustrated conventionally, anyof which may serve moving the wire along its course. A common supplypipe has been shown for the rinsing baths, but it will be understoodthat separate supply sources may be provided if desirable.

To review the effects of the different steps in the process briefly, itwill be recalled that the purpose of bath I 4 is to free the tungstenwire from its oxides and other surface impurities and to deposit thereona very thin adherent coating of nickel. This thin flash coating isapplied whilethe wire is in a clean condition and lays the basis for thesubsequent deposit of the heavier nickel plate. This heavier plate islaid on during the next step while the wire is passing through the bathI9. For the values given it is estimated that the nickel plate depositedin the bath I9 is about 0.00001 inch in thickness. The purpose of usingnickel for the initial plate is that it is one of the very few metalsthatadheres to tungsten. On the other hand, nickel tarnishes whenexposed to oxygen and will not, therefore, meet the requirements of therectifier contact wire if used as a soldering surface. Hence the nextstep in the process is to apply a relatively thick plating of gold overthe nickel plate. Gold was selected for two controlling reasons. In thefirst place it is highly resistant to oxidation and affords an excellentsurface for making the soldered joint between the contact wire and themetal holder of the rectifier. In the second place, gold alloys ordiffuses with nickel at a temperature sufficiently low to permit theheat treatment without injury to the tungsten wire. Finally the platedtungsten wire is given the heat treatment described, which results in apartial diffusion of the nickel plate into the body of the tungsten anda partial diffusion of the nickel plate into the outer plating of gold.The result, therefore, is an intimate adherent coating on the tungstenwire which satisfies the exacting requirements of these translatingdevices and which is capable of being formed into the necessary shapeswithout flaking or peeling.

Although specific values are given herein for the rates of movement ofthe wire through the various steps of the process and for the timeintervals of immersion, it will be understood that 6' these values maybe varied somewhat without departing from the invention.

After the tungsten wire is prepared as above described, a, length l iscut therefrom and soldered into the bore in the end of the brasscylindrical holder 3. The wire I is then shaped as illustrated in Fig.3, and the unit is now ready to be included in the assembly shown inFig. 4. The rectifier assembly includes a ceramic cylinder 31, theinterior of which is threaded to receive threaded studs 38 and 39. Stud38 is an integral part of the metal base 40 which closes one end of thehollow cylinder 31. The stud 38 also serves as a mounting for the thincrystal wafer 2 of silicon. The other stud 39 is an integral part of themetal cap 42, which contains a central bore 43 for receiving the holder3. The holder 3 is slipped into the bore 43 and adjusted by means of aset screw 44 until the tip end of the contact wire I engages the surfaceof the crystal 2 with the requisite degree of force. The cylinder 3 isthen seized by the set screws 45.

What is claimed is:

1. The method of preparing contact wire for point-contact translatorswhich comprises moving an indefinite length of tungsten wirecontinuously at a uniform rate, immersing the wire in an electricallyenergizing bath containing an etching agent for the wire, said wirebeing made the anode in the bath, immersing the moving wire for apredetermined interval in an electrolytic bath including a nickelcompound and an acid for cleaning the wire and giving it a thin initialadherent plating of nickel, said wire being made the cathode in saidelectrolytic bath, immersing said initially plated moving wire for apredetermined longer interval in an electrolytic nickel plating bath fordepositing thereon a plating of nickel of substantial thickness,immersing said moving wire for a still longer predetermined interval inan electrolytic gold plating bath for depositing on said nickel platinga plating of gold of substantial thickness, and subjecting said platedwire to a heat treatment sufficient to cause a diffusion of the initialnickel plating into the tungsten body of the wire and only a partialdiffusion between the outer plating of gold and the plating of nickel,

2. The method of preparing contact wire for rectifiers which comprisesmoving an indefinite length of tungsten wire at a predetermined ratethrough an electrically energized cleaning bath containing an agentsuitable for cleaning and etching the wire, said wire being made theanode in the cleaning bath, rinsing said wire, moving said wire throughan electrolytic bath including a nickel salt and hydrochloric acid forcleaning the wire and giving it a strike coating of nickel, said wirebeing made the cathode in said electrolytic bath, rinsing said wire,moving said coated wire at a predetermined rate through a secondelectrolytic nickel plating bath for depositing on said strike coating aplating of nickel of substantial thickness, moving said plated wire at apredetermined rate through a third electrolytic bath containing gold fordepositing on said nickel plating a plating of gold of substantialthickness, and subjecting the said plated wire to a heat treatment of710 C. to cause a portion only of the nickel plating to alloy with thetungsten body of the wire and to cause a portion only of the goldplating to alloy with the nickel plating to cause a portion only of thenickel plating to alloy with the tungsten body of the wire and.

to'cause a portion only of the gold plating to alloy with the nickelplating.

3. The method of preparing contact wire for point-contact rectifierswith a plating of metal which is free from flaking and peelingcomprising moving an indefinite length of wire continuously, passing thewire through en electrically energized cleaning bath containing asolution of sodium hydroxide, the wire being the anode in said cleaningbath, immersing the moving wire for an interval of twenty-five secondsin an electrolytic bath including a solution of nickel salt andhydrochloric acid for cleaning the wire and giving it a strike coatingof nickel, the wire being the cathode in said electrolytic bathimmersing said moving wire for an interval of sixty seconds in a secondelectrolytic nickel plating bath for depositing on said strike coating aplating of nickel of substantial thickness, immersing said moving wirefor an interval of two hundred and forty seconds in an electrolytic bathcontaining gold for depositing on said nickel plating a plating of gold,subjecting said plated wire to a heat treatment sufiicient to establisha close adherence between the wire and the strike coating of nickel andbetween the :platings of nickel and .gold thereon, but not sufficient tomake the platings of nickel and gold lose their identities, cutting saidtreated wine into unit lengths and bending said wire units to 1 givethem resilience when assembled in said rectifiers.

'4. The method of preparing contact wire for point-contact rectifierswhich comprises moving an indefinite length of tungsten wirecontinuously at a uniform linear speed, immersing said wire for apredetermined interval of time in a cleaning bath'of sodium hydroxide,said cleaning bath being electrically energized and having a cathodeandsaid wire being the anode, said pre-- determined interval of time beingsufdcient for cleaning and etching the tungsten wire, rinsing thecleaned and etched wire, immersingthe moving wire for a secondpredetermined time interval in an electrolytic bath including a nickelsalt and an acid for cleaning the wire, said second; predetermined timeinterval being sufificient forfurther cleaning said wire and giving it astrike coating of nickel, the cleaned and etched wire being the cathodein said electrolytic bath, rinsing the strike coated wire, immersing themoving wire for a longer predetermined interval in, a secondelectrolytic nickel plating bath for depositing on the strike coating aplating of nickel of substantial thickness, rinsin the nickel plated,wire, immersing the moving wireior a still'lo'nger predeterminedinterval in an electrolytic gold plating bath for depositing on theplatedwire a plating of gold of still greater thickness, rinsing thegold plated wire and heat-treating said plated wire in an atmosphere ofhydrogen at a temperature only sufficien-t to cause a portion of saidplating of nickel to difiuse into the tungsten body of the-wire andanother portion of the nickel to difiuse into the plating of gold butwithout &1- loylng the metal on the outer surface of said platingofgold.

5. The method of preparing electrical contact wire which comprisesmoving an indefinite length of wire at a predetermined uni-form linearspeed, passin the wire as an anode through an electrically energizedcleaning and etching bath containing a solution or sodium hydroxide,rinsing the wire, leading said wire as a cathode through an electrolyticbath including a solution of nickel salt and hydrochloric acid forcleaning the wire and giving it a strike coating of nickel, the nickelsalt being greater than the hydrochloric acid in the proportion of about2 to 1, rinsing the strike coated wire, leading said coated wire througha, second electrolytic nickel plating bath tfor depositing on saidstrike coating a plating of nickel .of substantial thickness, rinsingthe platedwire, leading said plated wire through a third electrolyticbath containing gold for depositin on said nickel plating an outerplatin of gold .of substantial -thickness, rinsing the gold plated wire,:and subjecting said plated wire to arheat'treatment of "710 C. to causediffusion 'between .a portion of the nickel plating and the tungstenbody of said wire and ,difiuslon between another portion of :said nickelplating and a p0r tion1=only of the gold of said outer plating to cause{diffusion between a portion or the nickel plating and .the tungstenbody .of said wire and diffusion eb-etween another portion of saidnickel plating and a portion only of the gold of said outer plating.

ROBERT A. EHRHARDT. RQBERT G. HUMBI-IREY.

REFERENCES CITED The following references are of record in the file ofthis patent:

STATES PATENTS

